字符串函数的模拟实现

目录

strlen

strcat

strcmp

strcpy

 strstr

---

strlen

实现：

#include <stdio.h>
#include <assert.h>
size_t my_strlen(const char* str)
{
    assert(str);
    size_t t = 0;
    while(*(str + t))
    {
        t++;
    }
    return t;
}

int main()
{
    char arr[] = "abcdef";
    int len = my_strlen(arr);
    printf("%d\n",len);
    return 0;
}

运行结果：

strcat

 返回destination

实现：

#include <stdio.h>
#include <assert.h>

char* my_strcat(char* dst, const char* src)
{
    assert(dst&&src);
    int t = 0;
    char* p = dst;
    while(*p)
    {
        p++;
    }
    while(*(p + t) = *(src + t))
    {
        t++;
    }
    return dst;
}

int main()
{
    char arr1[20] = "abcdef";
    char arr2[] = "123456";
    printf("%s\n",my_strcat(arr1,arr2));
    return 0;
}

 运行结果:

strcmp

 实现：

#include <stdio.h>
#include <assert.h>

int my_strcmp(const char* str1, const char* str2)
{
    assert(str1 && str2);
    while(*str1 == *str2 && *str1 != '\0')
    {
        str1++;
        str2++;
    }
    return *str1 - *str2;
}

int main()
{
    char str1[] = "abcdef";
    char str2[] = "abcdefg";
    char str3[] = "abcdef";
    char str4[] = "abcde";
    printf("%d\n",my_strcmp(str1,str2));
    printf("%d\n",my_strcmp(str1,str3));
    printf("%d\n",my_strcmp(str1,str4));
    
    return 0;
}

运行结果:

 strcpy

 实现:

#include <stdio.h>
#include <assert.h>

char* my_strcpy(char* dest, const char* src)
{
    assert(dest && src);
    char* p = dest;
    while(*p++ = *src++);
    return dest;
}


int main()
{
    char arr1[10] = "abcde";
    char arr2[] = "123456";
    printf("%s",my_strcpy(arr1,arr2));
    return 0;
}

运行结果:

 strstr

 实现1(bf)：

#include <stdio.h>
#include <assert.h>
#include <string.h>
#include <stdlib.h>


char* my_strstr(const char* s1, const char* s2)
{
    assert(s1 && s2);
    int n1 = strlen(s1);
    int n2 = strlen(s2);
    int j = 0;
    for(int i = 0; i < n1 - n2; i++)
    {
        int k = i;
        while(j < n2 && s1[k] == s2[j])
        {
            k++;
            j++;
        }
        if(j == n2)
        {
            return s1 + i;
        }
        j = 0;
    }
    return NULL;
}

实现2(KMP)

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <assert.h>

int* getNext(const char* s, int n)
{
    int* next = (int*)malloc(sizeof(int) * n);
    int k = -1;
    int j = 0;
    next[0] = k;
    while(j < n - 1)
    {
        if(k == -1 || s[j] == s[k])
        {
            k++;
            j++;
            next[j] = k;
        }
        else
        {
            k = next[k];
        }
    }
    return next;
}
//优化求next数组
int* getNext2(const char* s, int n)
{
    int* next = (int*)malloc(sizeof(int) * n);
    int k = -1;
    int j = 0;
    next[0] = -1;
    while(j < n - 1)
    {
        if(k == -1 || s[j] == s[k])
        {
            j++;
            k++;
            if(s[j] == s[k])
            {
                next[j] = next[k];
            }
            else
            {
                next[j] = k;
            }
        }
        else
        {
            k = next[k];
        }
    }
}

char* my_strstr(const char* s1, const char* s2)
{
    int n1 = strlen(s1);
    int n2 = strlen(s2);
    int* next = getNext(s2, n2);
    int i = 0;
    int j = 0;
    while(i < n1 - n2 + j)
    {
        if(j == -1)
        {
            i++;
            j++;
        }
        while(s1[i] == s2[j] && j < n2)
        {
            i++;
            j++;
        }
        if(j == n2)
        {
            free(next);
            return s1 + i - j;
        }
    }
    free(nexr);
    return NULL;
}